To most this might not seem all that useful, but it's my first PCB design, and we certainly have use for it so I thought I might as well post up. I've designed a custom Arduino much in the vein of the RBBB made by ModernDevice but with a few added features. The board is double layered, uses many SMD components, adds support for USB and is very electrically sound by comparison. Most importantly it maintains 3/4" DIP compatibility. There is more decoupling, and the use of ground planes help clean up signals.
http://thecodebenders.squarespace.com/journal/2009/2/18/and-we-decided-to-do-things-our-way.html
Hope you enjoy!
Eagle files are available